As explained in this post applying size to a wall or ceiling prior to papering prevents wallpaper shrinkage. This avoids the horrid gaps that can appear between each hung length once the paste has dried.
With new plaster the shrinkage problem increases because new plaster will suck the life out of wallpaper if the plaster isn’t sealed before doing anything with it.
So, you have your new bare plaster wall or ceiling. It needs to be sealed using either a sloppy mix of Dulux Supermatt or a solution of bog standard white matt paint with water added to make it sloppy – more detail in this post.
Paint your sloppy solution all over your new plaster.
You will notice that the sloppy solution dries incredibly quickly on the new plaster. That is because the plaster has sucked all the moisture out of the sloppy solution. Exactly what it would do to your lovely wallpaper, resulting in an awful finish.
Once dry, paint on another coat of the sloppy solution.
Allow to dry overnight.
Next day, apply wallpaper size to the wall.
Again, allow to dry overnight.
Your newly plastered walls or ceilings are now prepped and ready for wallpaper.